Pulse width modulation (PWM) is a common technique used to control the average speed of brushless direct current (BLDC) motors. Direct current motors have a characteristic that their speed is nearly linearly proportional to the average voltage applied across the motor windings. In a PWM controlled motor, a PWM waveform is used to control switches connecting the motor windings to a power supply. This is done in such a way that the average voltage across the motor is equal to the PWM duty cycle multiplied by the voltage of the power supply. Linear voltage regulators are also used to vary the DC voltage across BLDC motors; but they have the disadvantage of power losses across their pass elements.
PWM control signals are pulse trains where the duty cycle of the signal is defined as the average time high divided by the period. The period of the PWM control signal is typically chosen to be much smaller than the rotational period of the motor. Thus, many PWM pulses may occur within one rotation of the motor; and the pulses of equal width (duration) are evenly distributed throughout the motor rotation.
Torque and motion in BLDC motors are a result of the attraction and repulsion between the magnetic poles on the rotor and stator. The torque is a function of the magnetic field strength between the rotor and stator poles, as well as the geometry of the motor, and the relative angular position between the rotor and stator. In BLDC motors the magnetic pole orientation (and current) of the windings is reversed using commutation switches which replace the split ring commutator found in brushed motors. In order to avoid shorting out the power supply, there is a short commutation delay between when the winding current flowing in one direction is turned off, and the time before current is turned on in the other direction. This is analogous to the gap in the split ring commutator of brushed motors.
Applying a uniform PWM pulse train to control the speed of a BLDC motor has several disadvantages. For example, the energy is applied in the same manner throughout one rotation of the rotor and therefore fails to recognize physical differences in the motor that are related to rotor position. Thus, applying power to a motor in this manner can produce non-uniform torque output from the motor, increased audible noise from the motor, and energy losses that are higher than is desirable.
The inventor herein has recognized the above-mentioned disadvantages and has developed a method for improving motor control. In one embodiment of the present description, a method for controlling a brushless direct current motor, comprising: generating a motor command signal during a commutation interval of said brushless direct current motor; and adjusting a common attribute of at least two non-zero pulses of said motor command signal such that said at least two non-zero pulses of said motor command signal are not uniform.
By adjusting the width of pulses that make up a motor command signal during a commutation period of a motor cycle, operation of a motor may be improved. In particular, adjusting pulse widths of individual pulses that form a pulse train delivering energy to a motor can improve motor efficiency, reduce motor audible noise, and improve torque output characteristics of the motor. In one example, the first few pulses that are included in the pulse train of a motor commutation interval have a shorter duration (e.g., the amount if time a signal is high is shorter than the amount of time the signal is low during the period of a pulse). As the commutation interval continues, the pulses increase in duration for a time during the motor commutation interval. Finally, the pulse duration is decreased near the end of the motor commutation interval. This pulse train generating strategy can account for the position of motor poles during motor rotation in adjusting the pulse width so that attraction and repulsion between motor poles is improved and made more efficient, at least during some conditions.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.